Wire and cable insulations can be crosslinked with high voltage electron beam (EB) radiation. A great starting place would be this web site:
www.iba-worldwide.com/rdi/
Check out the RDI Menu on the right hand side and read the sections "Applications", "Science", and "E-Beam Theory"

Before high voltage electron beam machines, gamma radiation was sometimes used but that is now pretty much a dead issue for wire and cable.